High-voltage generators for X-ray tube power supplies as used in medical X-ray imaging typically comprise at least one multi-phase high-voltage transformer which provides the required power for operating the X-ray tube to the tube's cathode and anode. In conventional high-voltage generators circuits, an AC voltage adjusting device, such as e.g. an autotransformer, supplies line power to the multi-phase primary of a high-voltage transformer. A switching device, such as e.g. a silicon-controlled rectifier (SCR) in conjunction with a bridge rectifier, opens and closes the star point of the multi-phase primary to turn on and off high voltage at the X-ray tube. Inductive and capacitive effects in the transformer and associated power supply components generally cause the high voltage to rise above its steady-state level during a period immediately following completion of the circuit. The severity of this overshoot is known to increase with increasing X-ray tube voltage and to decrease with increasing X-ray tube current. Especially phase-shifted pulse width modulation (PWM) inverter-fed DC/DC power converters with a high-voltage transformer parasitic resonant link as used for an X-ray power generator thereby exhibit stiff nonlinear characteristics due to phase-shifted voltage regulation and diode cutoff operation in a high-voltage rectifier because of the wide load setting ranges in practical applications.
Modern resonant DC/DC converters, such as those used within high-voltage generator circuitries for providing X-ray tubes with high supply voltages, are operated at high switching frequencies. It is evident that any switching losses incurred by the associated power switches in a single switching cycle have to be reduced in order to limit the overall power losses.
An established method of doing this is zero current switching (ZCS), where turning the power switches on and off is only permitted at or near the zero crossings of the resonant current. This method is common practice in soft switching converters but has the drawback that it inhibits the controllability of the output power. Zero current switching and good controllability are effectively conflicting requirements, as good controllability is normally achieved by a continuous on-time control of the power switches, in which case ZCS can not be guaranteed for all points of operation. Therefore, switching losses emerge, such that there tends to be a trade-off between switching losses and controllability of the output power.
Recently, a variety of switched-mode high-voltage DC power supplies using voltage-fed-type or current-fed-type high-frequency transformer resonant inverters with MOS gate bipolar power transistors (IGBTs) have been developed for medical-use X-ray high power generators. In general, high-voltage high-power X-ray generators using voltage-fed high-frequency inverters in conjunction with a high-voltage transformer link have to meet the following requirements: (i) short rising period in start transient of X-ray tube voltage, (ii) no overshoot transient response in tube voltage, and (iii) minimized voltage ripple in periodic steady-state under extremely wide load variations and filament heater current fluctuation conditions of the X-ray tube.